1. Field of the Invention
This invention relates to the calibration of transmit antenna arrays.
The invention relates particularly to phased antenna arrays in which the transmitter connected to each antenna of the array includes phase shifters, to enable the individual antennas to be fed at different phases relative to each other in order to steer the transmitted beam.
2. Background Information
The inventors were concerned with antenna arrays for high frequency (HF) radar and in particular log periodic dipole arrays, an example of which (with four elements and five dipoles to an element) is shown in FIG. 1. The series of dipoles D forming each element 1, 2, 3, 4 of the array were fed at the front end, parallel wires W conducting the energy to the dipoles behind, and the radiating signal appeared along the boresight B of the element. The elements could be energized at different frequencies, and only certain dipoles within each element were energized for any given frequency. While investigating such antennas, the applicants made the discovery that, for any given element of the array, the center from which radiation appeared to be propagated, the so-called active region or phase center did not coincide exactly with the particular dipole that was being energized, but could instead be displaced from the center of the dipole, either along the direction of the element, or transverse to that direction, or both.